Circuit arrangements for starting and operating discharge lamps are used in electronic operating devices for discharge lamps. The starting of the discharge lamps is understood hereafter as meaning a preheating of electrode filaments of the discharge lamps during a preheating phase and an ignition of the discharge lamps during an igniting phase. The starting of discharge lamps with a preheating phase and an igniting phase is also known in English as Program Start. The igniting phase follows an operating phase in which the discharge lamp has an arc discharge.
According to the prior art, an electronic operating device for discharge lamps with Program Start requires a circuit arrangement which comprises a control unit that controls the course and sequence of the preheating, igniting and operating phases.
Circuit arrangements with an inverter which feeds energy into one end, respectively, of the electrode filaments via a matching network are known. The other ends, respectively, are connected via a resonant capacitor. The resonant capacitor and a lamp inductor are part of a resonant circuit, which has a resonant frequency which in the undamped case lies at the natural frequency. The matching network is required to transform the source resistance of the inverter into a source resistance of the operating device that is suitable for the operation of discharge lamps. Said resonant circuit is generally a component part of the matching network.
The inverter generates at an inverter output an inverter voltage with an inverter frequency which in a preheating phase lies at a high preheating frequency that is greater than the natural frequency. The value of the resonant capacitor and of the preheating frequency are chosen in such a way as to produce a heating current through the electrode filaments that brings about adequate preheating for the respective type of lamp.
After the preheating phase, the inverter frequency is lowered in an igniting phase until it is close enough to the natural frequency to produce at a connected discharge lamp an ignition voltage that brings about an ignition of the discharge lamp.
The ignition of the discharge lamp is followed by an operating phase. In this phase, controlled variables, such as for example lamp power or lamp current, are fed to a controller. The controller uses a manipulated variable to act on the inverter frequency in such a way that a desired lamp power or a desired lamp current is produced.
The described prior art is described in various embodiments in the following documents:    EP 0 845 928 (Mita)    EP 0 930 808 (Kanazawa)
In the prior art, a control unit which sets the required inverter frequency in the correct time sequence in the respective phases is required. Moreover, the control unit must deactivate the control of the lamp power or lamp current during the preheating and igniting phases, since an inverter frequency which does not depend on the lamp power or lamp current is required in these phases.
With increasing cost pressure in respect of the operating devices for discharge lamps with which the invention is concerned, it is becoming increasingly important to dispense with parts of these operating devices.